This invention concerns an offset printing plate and more particularly it relates to direct use of a silver image obtained by silver complex diffusion transfer process as an offset printing plate. That is, this invention relates to improvement of negative-type direct offset printing plates disclosed in U.S. Pat. No. 3,728,114, etc.
Negative-type direct offset printing plates can be advantageously obtained using direct-positive silver halide emulsions as silver halide photographic emulsions. Such negative-type direct offset printing plates are disclosed in Japanese Patent Applications No. 22829/76 and No. 27095/76. The former shows that the adhesion strength of silver image increases with increase in proportion of chloride in the silver halide emulsion and that emulsions comprising silver chlorobromide or silver chloroiodobromide grains containing at least 50 mol % of chloride, surfaces of which are substituted with not more than 2% of iodide are preferred for obtaining negative-type direct offset printing plate having high photographic speed and desirable printability. The latter shows that in order to obtain negative-type direct offset printing plates of higher photographic speed it is preferred to use silver halide emulsions comprising silver chlorobromide, silver iodobromide, silver chloroiodobromide or silver bromide grains which contain at least 80 mol % and furthermore for preventing reduction of printabilities it is preferred to limit the weight ratio of binder and silver to 1/5-2 and the coating amount of silver to 0.5-2.5 g/m.sup.2. Moreover, Japanese Patent Application No. 95274/76 discloses that use of cationic cyanine dyes causes inhibition of diffusion transfer development and reduction in the deposition amount of silver while use of betaine type or anion type cyanine dyes results in prevention of reduction in the deposition amount of silver and preferred printabilities. Thus improved negative-type direct offset printing plates have fairly high photographic speed and fairly improved printabilities, but amount of transferred silver is still not sufficient, running strength tends to vary at every plate and is low and especially reproducibility of fine lines is not sufficiently high.